Generally, handovers in communication systems are important research topics. When a mobile station decides to conduct a handover, the mobile station transmits a handover request message to the serving base station. In response to the handover request, the base station transmits a handover acknowledgement (ACK) message to the mobile station. After receiving the handover ACK message, the mobile station determines a target base station, to which a handover is to be conducted, and transmits a handover indication message, which contains information regarding the determined target base station, to the serving base station.
A handover procedure in a communication system will now be described with reference to FIG. 1.
FIG. 1 is a flowchart showing a handover procedure in a communication system according to the prior art.
Referring to FIG. 1, the mobile station (MS) 100 receives a MOB_NBR-ADV (mobile neighbor advertisement) message from the serving base station 120, from which the MS 100 is currently receiving a service. After receiving the MOB_NBR-ADV message, the MS 100 conducts a scanning operation for measuring the signal intensity with regard to the serving base station 120 and target base stations 140 and 160 (step 103). After completing the scanning operation, the MS 100 transmits a MOB_MSHO-REQ (mobile mobile station handover request) message to the serving base station 120 (step 105).
The serving base station 120 transmits HO_notification (handover notification) messages to the first and second target base stations 140 and 160 to inform that the MS 100 may hand over (steps 107 and 109).
The first and second target base stations 140 and 160 transmit messages containing responses to the HO_notification messages to the serving base station 120 (steps 111 and 113).
The serving base station 120 transmits a MOB_BSHO-RSP (mobile base station handover response) message, which contains information regarding services the target base stations 140 and 160 can provide, to the MS 100 (step 115).
The MS 100 transmits a MOB_HO-IND (mobile handover indication) message to the serving base station 120 to inform that the MS 100 will hand over to the second target base station 160 (step 117).
After receiving the MOB_HO-IND message, the serving base station 120 transmits a HO_confirm (handover confirmation) message to the second target base station 160 to inform that the MS 100 will hand over to the second target base station 160 (step 119).
After receiving the MOB_HO-IND message, the serving base station 120 stops transmitting packets to the MS 100. In addition, the serving base station 120 adds information regarding the session of the MS 100 to the HO_confirm message and transmits it so that the MS 100 is continuously provided with the service from the second target base station 160 after completing the handover.
In some cases in which the MS can not transmit MOB_MSHO-REQ or MOB_HO-IND to the serving base station, the MS may proceed with network reattachment process at the target base station without any pre-notification. In this case, the target base station shall retrieve MS status information from the serving base station via backhaul communication.
Meanwhile, ARQ (Automatic Repeat Request) refers to a scheme for detecting the loss of frames in MAC (Medium Access Control) layers and retransmitting packets accordingly. Lost frames can be detected from polling or acknowledgements received from the retransmission timer or receiving side. The ARQ scheme can quickly restore lost frames and efficiently guarantee the reliability of radio links.
When the communication system adopts the ARQ scheme, the second target base station 160 must retransmit a packet (i.e., ARQ block) if the serving base station 120 fails to receive an acknowledgement in connection with the packet. In this case, the serving base station 120 must transmit ARQ status information regarding the packet transmission status and the ARG block stored in the buffer to the second target base station 160 so that the second target base station 160 can retransmit necessary ARQ blocks.
One of the easy methods for transmitting the ARQ status information is to transmit status information consisting of every detailed transmission status of each packet to the second target base station 160 by the serving base station 120. In this case, the information regarding the status of each packet may be of MAP type, including paired values of packet sequence numbers and transmission status variables. However, this method has a problem in that the size of the status information increases rapidly in proportion to the number of packets in the buffer and the number of flows. In addition, the fact that the second target base station 160 must conduct 1-to-1 remapping between the transmitted status information and each packet requires a long period of time to restore the status information.